1. Field of the Invention
The present invention relates to an image processing apparatus having a solid-state image pickup device used in a digital color copying machine, a color facsimile or an image file, and more particularly to a color image processing apparatus having an improved color balance for a read image data.
2. Description of the Prior Art
A digital color copying machine which uses an image read unit having a solid-state image pickup device such as a charge coupled device (CCD) and processes an electrical signal to reproduce an image by a digital printer such as a laser beam printer or an ink jet printer has been known. In such an apparatus, a color image read unit as shown in FIG. 1 is commonly used. Referring to FIG. 1, light reflected by an original 2 illuminated by an illumination lamp 1 is focused onto a solid-state image pickup device 7 comprising a color decomposing filter 5 and a CCD 6, through a mirror 3 and a lens 4 to constitute an image data.
The image data thus provided is sequentially transferred one line at a time in synchronism with a clock pulse from an oscillator 8 as shown in FIG. 2, amplified by an amplifier 9, and converted to a digital image data of a predetermined number of bits for each pixcel by an analog-to-digital converter 10. The digital image data is than supplied to a dither circuit 11, which compares it with a predetermined dither pattern stored in a ROM to encode the digital data. The encoded data is temporarily stored in a buffer memory 12.
In the prior art image read unit, the color decomposing filter, 5 comprises three filters one for each of the three principal colors R, G and B, and the image data of the respective principal colors are read through the respective filters. In order to improve tonality of a reproduced color image or to improve a color balance, different dither patterns are used in the dither circuit 11 of the read unit of FIG. 2 for the respective principal colors, or the light intensity of the illumination lamp 1 is changed for each of the principal colors. However, since the complex dither patterns are required, the selection of a proper dither pattern is difficult to attain and fine control of the light intensity is also difficult to attain. Accordingly, a satisfactory improvement is not attained.